Re: [RFC 7/8] blkio: implement BDRV_REQ_REGISTERED_BUF optimization

[Stefan Hajnoczi]

On Tue, Mar 29, 2022 at 05:24:30PM +0200, Stefano Garzarella wrote:
> On Wed, Mar 23, 2022 at 11:17:26AM +0000, Stefan Hajnoczi wrote:
> > Avoid bounce buffers when QEMUIOVector elements are within previously
> > registered bdrv_register_buf() buffers.
> > 
> > The idea is that emulated storage controllers will register guest RAM
> > using bdrv_register_buf() and set the BDRV_REQ_REGISTERED_BUF on I/O
> > requests. Therefore no blkio_add_mem_region() calls are necessary in the
> > performance-critical I/O code path.
> > 
> > This optimization doesn't apply if the I/O buffer is internally
> > allocated by QEMU (e.g. qcow2 metadata). There we still take the slow
> > path because BDRV_REQ_REGISTERED_BUF is not set.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > block/blkio.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 104 insertions(+), 4 deletions(-)
> > 
> > diff --git a/block/blkio.c b/block/blkio.c
> > index dd2308b967..78f4ca5f49 100644
> > --- a/block/blkio.c
> > +++ b/block/blkio.c
> > @@ -1,7 +1,9 @@
> > #include "qemu/osdep.h"
> > #include <blkio.h>
> > #include "block/block_int.h"
> > +#include "exec/memory.h"
> > #include "qapi/error.h"
> > +#include "qemu/error-report.h"
> > #include "qapi/qmp/qdict.h"
> > #include "qemu/module.h"
> > 
> > @@ -26,6 +28,9 @@ typedef struct {
> >     /* Can we skip adding/deleting blkio_mem_regions? */
> >     bool needs_mem_regions;
> > 
> > +    /* Are file descriptors necessary for blkio_mem_regions? */
> > +    bool needs_mem_region_fd;
> > +
> >     /*
> >      * blkio_completion_fd_poll() stashes the next completion for
> >      * blkio_completion_fd_poll_ready().
> > @@ -170,6 +175,8 @@ static BlockAIOCB *blkio_aio_preadv(BlockDriverState 
> > *bs, int64_t offset,
> >         BlockCompletionFunc *cb, void *opaque)
> > {
> >     BDRVBlkioState *s = bs->opaque;
> > +    bool needs_mem_regions =
> > +        s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF);
> >     struct iovec *iov = qiov->iov;
> >     int iovcnt = qiov->niov;
> >     BlkioAIOCB *acb;
> > @@ -179,7 +186,7 @@ static BlockAIOCB *blkio_aio_preadv(BlockDriverState 
> > *bs, int64_t offset,
> > 
> >     acb = blkio_aiocb_get(bs, cb, opaque);
> > 
> > -    if (s->needs_mem_regions) {
> > +    if (needs_mem_regions) {
> >         if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) {
> >             qemu_aio_unref(&acb->common);
> >             return NULL;
> > @@ -194,7 +201,7 @@ static BlockAIOCB *blkio_aio_preadv(BlockDriverState 
> > *bs, int64_t offset,
> > 
> >     ret = blkioq_readv(s->blkioq, offset, iov, iovcnt, acb, 0);
> >     if (ret < 0) {
> > -        if (s->needs_mem_regions) {
> > +        if (needs_mem_regions) {
> >             blkio_free_mem_region(s->blkio, &acb->mem_region);
> >             qemu_iovec_destroy(&acb->qiov);
> >         }
> > @@ -215,6 +222,8 @@ static BlockAIOCB *blkio_aio_pwritev(BlockDriverState 
> > *bs, int64_t offset,
> > {
> >     uint32_t blkio_flags = (flags & BDRV_REQ_FUA) ? BLKIO_REQ_FUA : 0;
> >     BDRVBlkioState *s = bs->opaque;
> > +    bool needs_mem_regions =
> > +        s->needs_mem_regions && !(flags & BDRV_REQ_REGISTERED_BUF);
> >     struct iovec *iov = qiov->iov;
> >     int iovcnt = qiov->niov;
> >     BlkioAIOCB *acb;
> > @@ -224,7 +233,7 @@ static BlockAIOCB *blkio_aio_pwritev(BlockDriverState 
> > *bs, int64_t offset,
> > 
> >     acb = blkio_aiocb_get(bs, cb, opaque);
> > 
> > -    if (s->needs_mem_regions) {
> > +    if (needs_mem_regions) {
> >         if (blkio_aiocb_init_mem_region_locked(acb, bytes) < 0) {
> >             qemu_aio_unref(&acb->common);
> >             return NULL;
> > @@ -238,7 +247,7 @@ static BlockAIOCB *blkio_aio_pwritev(BlockDriverState 
> > *bs, int64_t offset,
> > 
> >     ret = blkioq_writev(s->blkioq, offset, iov, iovcnt, acb, blkio_flags);
> >     if (ret < 0) {
> > -        if (s->needs_mem_regions) {
> > +        if (needs_mem_regions) {
> >             blkio_free_mem_region(s->blkio, &acb->mem_region);
> >         }
> >         qemu_aio_unref(&acb->common);
> > @@ -286,6 +295,83 @@ static void blkio_io_unplug(BlockDriverState *bs)
> >     }
> > }
> > 
> > +static void blkio_register_buf(BlockDriverState *bs, void *host, size_t 
> > size)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    char *errmsg;
> > +    int ret;
> > +    struct blkio_mem_region region = (struct blkio_mem_region){
> > +        .addr = host,
> > +        .len = size,
> > +        .fd = -1,
> > +    };
> > +
> > +    if (((uintptr_t)host | size) % s->mem_region_alignment) {
> > +        error_report_once("%s: skipping unaligned buf %p with size %zu",
> > +                          __func__, host, size);
> > +        return; /* skip unaligned */
> > +    }
> > +
> > +    /* Attempt to find the fd for a MemoryRegion */
> > +    if (s->needs_mem_region_fd) {
> > +        int fd = -1;
> > +        ram_addr_t offset;
> > +        MemoryRegion *mr;
> > +
> > +        /*
> > +         * bdrv_register_buf() is called with the BQL held so mr lives at 
> > least
> > +         * until this function returns.
> > +         */
> > +        mr = memory_region_from_host(host, &offset);
> > +        if (mr) {
> > +            fd = memory_region_get_fd(mr);
> > +        }
> > +        if (fd == -1) {
> > +            error_report_once("%s: skipping fd-less buf %p with size %zu",
> > +                              __func__, host, size);
> > +            return; /* skip if there is no fd */
> > +        }
> > +
> > +        region.fd = fd;
> > +        region.fd_offset = offset;
> > +    }
> > +
> > +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +        ret = blkio_add_mem_region(s->blkio, &region, &errmsg);
> > +    }
> > +
> > +    if (ret < 0) {
> > +        error_report_once("Failed to add blkio mem region %p with size 
> > %zu: %s",
> > +                          host, size, errmsg);
> > +        free(errmsg);
> > +    }
> > +}
> > +
> > +static void blkio_unregister_buf(BlockDriverState *bs, void *host, size_t 
> > size)
> > +{
> > +    BDRVBlkioState *s = bs->opaque;
> > +    char *errmsg;
> > +    int ret;
> > +    struct blkio_mem_region region = (struct blkio_mem_region){
> > +        .addr = host,
> > +        .len = size,
> > +    };
> > +
> > +    if (((uintptr_t)host | size) % s->mem_region_alignment) {
> > +        return; /* skip unaligned */
> > +    }
> > +
> > +    WITH_QEMU_LOCK_GUARD(&s->lock) {
> > +        ret = blkio_del_mem_region(s->blkio, &region, &errmsg);
> > +    }
> > +
> > +    if (ret < 0) {
> > +        error_report_once("Failed to delete blkio mem region %p with size 
> > %zu: %s",
> > +                          host, size, errmsg);
> > +        free(errmsg);
> > +    }
> > +}
> > +
> > static void blkio_parse_filename_io_uring(const char *filename, QDict 
> > *options,
> >                                           Error **errp)
> > {
> > @@ -356,6 +442,18 @@ static int blkio_file_open(BlockDriverState *bs, QDict 
> > *options, int flags,
> >         return ret;
> >     }
> > 
> > +    ret = blkio_get_bool(s->blkio,
> > +                         "needs-mem-region-fd",
> > +                         &s->needs_mem_region_fd,
> > +                         &errmsg);
> > +    if (ret < 0) {
> > +        error_setg_errno(errp, -ret,
> > +                         "failed to get needs-mem-region-fd: %s", errmsg);
> > +        free(errmsg);
> > +        blkio_destroy(&s->blkio);
> > +        return ret;
> > +    }
> > +
> >     ret = blkio_get_uint64(s->blkio,
> >                            "mem-region-alignment",
> >                            &s->mem_region_alignment,
> 
> I already mentioned on IRC while testing the series, but I'm writing it here
> so we don't forget ;-)
> 
> To prevent bdrv_driver_pwritev() from removing the BDRV_REQ_REGISTERED_BUF
> flag from requests, we should add this:
> 
> @@ -474,7 +474,7 @@ static int blkio_file_open(BlockDriverState *bs, QDict 
> *options, int flags,
>          return ret;
>      }
> 
> -    bs->supported_write_flags = BDRV_REQ_FUA;
> +    bs->supported_write_flags = BDRV_REQ_FUA | BDRV_REQ_REGISTERED_BUF;
> 
>      qemu_mutex_init(&s->lock);
>      s->blkioq = blkio_get_queue(s->blkio, 0);

Thanks, will fix.

Stefan

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